Dedicated macrophages organize and maintain the enteric nervous system.

Correct development and maturation of the enteric nervous system (ENS) is critical for survival1. At birth, the ENS is immature and requires considerable refinement to exert its functions in adulthood2. Here we demonstrate that resident macrophages of the muscularis externa (MMϕ) refine the ENS early in life by pruning synapses and phagocytosing enteric neurons. Depletion of MMϕ before weaning disrupts this process and results in abnormal intestinal transit. After weaning, MMϕ continue to interact closely with the ENS and acquire a neurosupportive phenotype. The latter is instructed by transforming growth factor-ß produced by the ENS; depletion of the ENS and disruption of transforming growth factor-ß signalling result in a decrease in neuron-associated MMϕ associated with loss of enteric neurons and altered intestinal transit. These findings introduce a new reciprocal cell-cell communication responsible for maintenance of the ENS and indicate that the ENS, similarly to the brain, is shaped and maintained by a dedicated population of resident macrophages that adapts its phenotype and transcriptome to the timely needs of the ENS niche.

Microglial tissue surveillance: The never-resting gardener in the developing and adult CNS.

Immunosurveillance by microglia is a dynamic process in the central nervous system (CNS) with versatile functions to maintain tissue homeostasis and provide immune defense. A tightly controlled microglia network throughout the CNS parenchyma facilitates efficient immunosurveillance, where each cell guards a certain tissue territory. Each cell is constantly surveilling its environment and the surrounding cells, screening for pathogens but also removing cell debris and metabolites, grooming neighboring cells and facilitating cellular crosstalk. In the absence of inflammation, this "tissue surveillance" by microglia presents an essential process for CNS homeostasis and development. In this review, we provide a summary on different tissue surveillance functions mediated by microglia, the underlying molecular machineries, and how defects, such as genetic mutations, can alter these surveillance mechanisms and cause disease onset.

An improved organotypic cell culture system to study tissue-resident macrophages ex vivo.

Tissue-resident macrophages (TRMs) perform organ-specific functions that are dependent on factors such as hematopoietic origin, local environment, and biological influences. A diverse range of in vitro culture systems have been developed to decipher TRM functions, including bone marrow-derived macrophages (BMDMs), induced pluripotent stem cell (iPSC)-derived TRMs, or immortalized cell lines. However, despite the usefulness of such systems, there are notable limitations. Attempts to culture primary macrophages often require purification of cells and lack a high cell yield and consistent phenotype. Here, we aimed to address these limitations by establishing an organotypic primary cell culture protocol. We obtained long-term monocultures of macrophages derived from distinct organs without prior purification using specific growth factors and tissue normoxic conditions that largely conserved a TRM-like identity in vitro. Thus, this organotypic system offers an ideal screening platform for primary macrophages from different organs that can be used for a wide range of assays and readouts.

Microglia in a Dish-Which Techniques Are on the Menu for Functional Studies?

Microglia build the first line of defense in the central nervous system (CNS) and play central roles during development and homeostasis. Indeed, they serve a plethora of diverse functions in the CNS of which many are not yet fully described and more are still to be discovered. Research of the last decades unraveled an implication of microglia in nearly every neurodegenerative and neuroinflammatory disease, making it even more challenging to elucidate molecular mechanisms behind microglial functions and to modulate aberrant microglial behavior. To understand microglial functions and the underlying signaling machinery, many attempts were made to employ functional in vitro studies of microglia. However, the range of available cell culture models is wide and they come with different advantages and disadvantages for functional assays. Here we aim to provide a condensed summary of common microglia in vitro systems and discuss their potentials and shortcomings for functional studies in vitro.

CNS Macrophages and Infant Infections.

The central nervous system (CNS) harbors its own immune system composed of microglia in the parenchyma and CNS-associated macrophages (CAMs) in the perivascular space, leptomeninges, dura mater, and choroid plexus. Recent advances in understanding the CNS resident immune cells gave new insights into development, maturation and function of its immune guard. Microglia and CAMs undergo essential steps of differentiation and maturation triggered by environmental factors as well as intrinsic transcriptional programs throughout embryonic and postnatal development. These shaping steps allow the macrophages to adapt to their specific physiological function as first line of defense of the CNS and its interfaces. During infancy, the CNS might be targeted by a plethora of different pathogens which can cause severe tissue damage with potentially long reaching defects. Therefore, an efficient immune response of infant CNS macrophages is required even at these early stages to clear the infections but may also lead to detrimental consequences for the developing CNS. Here, we highlight the recent knowledge of the infant CNS immune system during embryonic and postnatal infections and the consequences for the developing CNS.